Recent test failures leads to suspicion that our variable packing algorithm is
buggy - turns out to be an underlying driver bug.

With the added test cases, such suspicion shouldn't even arise.

BUG=angleproject:1142
TEST=angle_unittests

Change-Id: I1fb3c5c7798d9ad17668a3d633286e031da79cab
Reviewed-on: https://chromium-review.googlesource.com/295901Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Tested-by: Zhenyao Mo <zmo@chromium.org>